In surface filtration, solids form a filter cake which is predominantly on the surface of a porous filter medium. Surface filtration is distinguishable from depth filtration, in which a significant portion of solids are trapped relatively deep within a filter medium. In surface and in depth filtration, the filter medium is often in the form of a cartridge, which can be backwashable or disposable. The filter medium can be fabricated of, for example, cotton, polypropylene, nylon, polyethylene, polyester, or metal wire cloth.
Precoat filtration is a specific category of filtration. Precoat filter elements utilize an applied filter medium upon a porous support structure, termed a septum, to form a filter cake that is substantially isolated from the septum. The precoat can be a granular material, such as diatomaceous earth, perlite or cellulose fibers. The septum can be a woven fabric, a yarn-wound element, a laid non-woven fabric, or a relatively rigid perforated element. The precoat performs a filtration function and, thereby, protects the septum from fouling.
Typically, the size of the precoat media is larger than the size of the septum pores. The septum functions primarily as a support member and the precoat media perform most of the filtration that occurs. Periodically, the precoat is removed by, for example, backwashing and another precoat is applied.
In addition to performing the filtration function, some precoats may also interact chemically with contaminants in a process stream. For example, precoats including ion exchange resins are described in U.S. Pat. Nos. 3,250,702, 3,250,703, 3,250,704 and 3,250,705 to Levendusky. A mixture of powdered cation and anion exchange resin is applied as a precoat having a thickness of about one-sixteenth to about two inches on the outside of filter cartridge elements in a pressure shell. The described ion exchange resins have a particle size range of about 100 to 400 mesh. A pressure differential across the filter precoated with a mixture of powdered cation and anion exchange resin is relatively low when the resin is clean. The improvement in pressure differential is attributed to electrostatic forces which tend to clump cation and anion exchange particles, making the mixture more permeable to liquid flow. Other materials, such as cellulose fibers have been blended with the mixture of cation and anion resins in order to make a flocculating mixture that is still more clumping and permeable.
A method for removing impurities which employs a clumping filter precoat of oppositely charged particles of filter aid material is described in U.S. Pat. No. 4,177,142. The described filter aid materials include diatomaceous earth, cellulose fibers, charcoal, expanded perlite, asbestos fibers, ion exchange resins, and inorganic ion exchangers. The filter bed is portrayed as a mixture of oppositely charged particles of filter aid material, some having a normal surface charge and others having a chemically induced surface charge. The precoat of oppositely charged particles exhibits the clumping phenomenon.
Specialized apparatus have been developed to better exploit the advantages of mechanical and ion exchange precoats. For example, UK Patent Application GB 2 214 447A discloses a filter element for a precoat filter having a septum with longitudinal pleats. Circumferential bands constrain the septum during backwashing. A precoat having a minimum thickness of about 5 mm (0.2 inch) is described. This application states that precoat-type filters are used with ion exchange resin coatings in the treatment of feedwater and condensate for nuclear power generating systems, and that the filters may be backwashed to dislodge the precoat.
Various types of ultrafiltration and microfiltration apparatus have been used to separate metallic corrosion products from water streams, particularly boiler feedwater makeup and condensate return streams. Ultrafiltration is a pressure driven filtration on a molecular scale that includes passing a liquid stream through a sheet filter or, alternatively, a hollow fiber having a pore size of about 0.01 micron. Microfiltration is directed at relatively larger colloids and suspended particles, employing sheet filters, such as membranes, cartridge filters, or pleated filters having a pore size of about 0.1 microns. Alternatively, hollow fibers having a pore size of about 0.1 microns can be employed for microfiltration.
Hollow fibers differ from sheet filter membranes in that the hollow fibers are tubes having internal diameters in the range of about 0.1 to about 1 mm. Sheet filters are usually flat sheets or spiral-wound membranes fabricated from sheets. However, hollow fiber microfilters and sheet microfilters are often constructed from the same materials, such as cellulose acetate, polyamide, polysulfone, polyacrylonitrile, polyfuran, nylon and polyethylene.
The sheet filter can be woven, laid, cast, wound, extruded, or hot-melted and sprayed, as compared hollow fibers which are usually produced by casting. Sheet filters can process a relatively higher flow of liquid per unit area of septum, and are generally more economical to install and to operate, than the hollow fibers.
Japanese Kokai Patent Application No. HE 02-83020 describes filtration of recirculating water by hollow fibers which have been coated with powdered cation exchange resins for removal of ion impurities such as ferric and cupric ions. The application states that iron oxides can optionally be used as the coating. It is believed that the use of cation exchange resins described in the application is directed to the removal of ions, rather than particulates.
A filtration process using a hollow fiber filter is described in Japanese Kokai Patent Application Nos. SH 062-87023; SH 062-220200; and HE 011-80205. Water containing fine particles of iron oxide is reportedly passed through the hollow fiber filter to form a thin coating of iron oxide or iron hydroxide on the membrane. The thin coating is said to facilitate backwashing of contaminants which are subsequently collected on the filter. Reportedly, the layer of iron oxides shields the membrane from ferrous ions in the recirculating stream and, thereby, prevents the ferrous ions from precipitating on the membrane surface.
Hollow fiber filters and sheet filters are well suited for use in nuclear power generating stations because they can be effectively backwashed while producing a relatively small amount of backwash waste water for radioactive waste disposal. With the hollow fiber filters, it is possible to fabricate filter elements having relatively more surface area per unit volume than with the sheet filters. On the other hand, the sheet filters exhibit relatively higher flux rates, lower initial cost, and lower replacement cost.
Significant factors in selecting a filter for use in a nuclear power generating plant, are the frequency of backwashing in a particular service, and whether the filter exhibits a relatively low pressure differential after backwashing. Filters with relatively small pores tend to accumulate iron corrosion products on their external surfaces, perhaps because their pores are smaller in size than many contaminants. In comparison, filters having relatively larger pores, tend to allow some of the contaminants, such as iron corrosion products, to penetrate beneath their surfaces. Filters having relatively larger pores can be more difficult to backwash completely so as to regain an original condition pressure differential.
Another significant selection factor can be the amount of material which is sent to disposal as a result of backwashing or discarding filter. Waste disposal is usually expensive. In some applications, used precoat media and spent backwash water must be treated as radioactive or hazardous waste.
A need exists for an improved method of filtration which removes iron corrosion products from a water stream at relatively high efficiencies and which produces a filter cake that can be dislodged so that the filter is returned approximately to its original clean condition. The improved method should provide relatively low initial pressure differentials, and also longer service cycles that are not interrupted by plugging or precoat exhaustion. The improved method must not produce unnecessary waste for disposal.